As two young women arrested in Peru on suspicion of cocaine trafficking face a potential three years in prison awaiting trial, Prisoners Abroad warned overcrowding is at such critical levels inmates' face serious health dangers.

If sentenced to prison, Melissa Reid, 19, of Scotland, and Michaella McCollum Connolly, 20, who has an Irish passport, could face years behind bars.

"People often have to sleep on floors, in corridors, or even in the toilet areas," a spokesman said.

"The diet is extremely poor. Without being able to buy other food to supplement their diet people are lost. Clean water is a rarity and has to be purchased, often at inflated prices.

"Because of the overcrowding, infectious diseases such as tuberculosis and HIV are constant concerns."

Peru's prison system is at 200% capacity.

Medication is not provided for chronic or critical conditions, and medical treatments are generally paid for by prisoners or their families.

Prisoners Abroad, which sends survival grants to Britons held in Peru, warned medical concerns and isolation make time in a Peruvian prison "hugely distressing".

The group, which does not comment on individual cases, said the experience is particularly harrowing for inmates who do not speak the language.

Meanwhile, legal expert Bruno Min said Ms McCollum Connolly and Ms Reid may have the possibility of seeing their jail term out in Britain eventually.

His organisation, Fair Trials International, works to protect the rights of people facing criminal charges in a country other than their own.

"We deal with hundreds of cases each year. Out of all of the cases we deal with in Latin America perhaps Peru might be one of the largest producers of our case load," assistance co-ordinator Mr Min said.

"The vast majority of cases that we have dealt with from Peru are drugs offences, possibly because it is a large producer of illegal drugs."

Drugs cases are treated slightly differently in Peru from other offences, he said, with people often ineligible for bail.

Ms Reid and Ms McCollum Connolly could be held pre-charge for up to 30 days, he said, and then could spend up to three years in prison before a trial.

"If they are charged they will be sent to prison before their trial goes ahead and we understand that the maximum period for pre-trial detention is 36 months," he said.

"If you are charged with a drugs offence, it is our understanding that you are very unlikely to get released before your trial takes place, you're almost ineligible for bail.

"There is the likelihood that they could end up spending 36 months in pre-trial detention."

He added that the justice system in Peru suffers from delays, and that drug trafficking offences are regarded as particularly serious.

With drug offence convictions, normal rules on early release do not apply.

Mr Min said there is a "prison transfer treaty" between Peru and the UK which means the women could apply for the right to finish serving their sentence in the UK.

Concerns for the women, as well as other accused people abroad, include their lack of support and the variation in legal systems, he said.

"Not every country in the world has the same standards of criminal justice and many are routinely criticised for breaches of basic rights, and that could also result in problems for these two girls," he said.